The New South Wales locality of Port Stephens, north of Sydney, has been declared a coronavirus hotspot by the Northern Territory's Chief Minister Michael Gunner.
The decision means anyone who has been in Port Stephens in the past 14 days will have to undergo two weeks of supervised quarantine upon arrival in the NT or return home.
In a statement on social media, Mr Gunner said the decision was needed to "keep Territorians safe".
"I know that's bad news for other Australians, but it's what needs to happen to keep Territorians safe, and that's my top priority," Mr Gunner said.
"As Territorians, we're all on the frontline now. And every single thing we do matters."
The NT reopened its borders on Friday, July 17, but all arrivals from coronavirus hotspots need to enter two weeks of mandatory supervised quarantine at their own cost of $2,500.
The Territory has already declared Victoria and Greater Sydney hotspots.
Three days ago it added the Eurobodalla Shire in New South Wales to the list of hotspots.
The announcement followed an outbreak of eight coronavirus cases linked to the Soldiers Club in Batemans Bay.
